To shed some light on the potential value of rifampicin in the treatment of tuberculous meningitis (TBM) in adults, a retrospective analysis has been made of 143 medical records from 4 hospitals for the period 1967-80. Treatment of TBM with rifampicin and other antituberculous drugs in combination (Group B) was compared to other regimes which did not include rifampicin (Group A). There were 64 patients in Group B and 79 in Group A. The two groups of patients did not differ significantly in their prognostic characteristics. The total mortality was 14.7%: it was higher among patients not treated with rifampicin (24%; Group A) than amongst those given rifampicin (3.1%; Group B; chi 2 = 10.74; p less than 0.005). The difference was also statistically significant (chi 2 = 6.88; p less than 0.01) if patients who died during the first 48 h after the institution of treatment were excluded. No significant difference in mortality rate was found when patients treated with rifampicin plus isoniazid (INH) 8-10 mg/kg (1 death out of 41 patients) were compared to patients treated with INH 15 mg/kg (2 deaths out of 20 patients). Neurological sequelae recorded during a 6 month follow-up period were more severe among patients not treated with rifampicin.